Multi-carrier modulation and Orthogonal Frequency Division Multiplexing (OFDM) in particular, are attractive technologies for broadband high data-rate communications due to their robustness against long delay spread and lower complexity when compared to single carrier systems. In addition to multi-carrier modulations, Adaptive Modulation/Coding (AMC) is also a fundamental technique for wireless broadband communications. With AMC, the modulation and coding scheme (MCS) of a transmitted data stream for a particular receiver is changed to predominantly match a current received signal quality (at the receiver) for the particular frame being transmitted. The received signal quality is determined by the channel quality. (The terms “received signal quality” and “channel quality” can be referred to interchangeably). The modulation and coding scheme may change on a frame-by-frame basis in order to track the channel quality variations that occur in mobile communication systems. Thus, streams with high quality are typically assigned higher order modulations and/or higher channel coding rates with the modulation order and/or the code rate decreasing as quality decreases. For those receivers experiencing high quality, modulation schemes such as 16-QAM, 64-QAM or 256-QAM are utilized, while for those experiencing low quality, modulation schemes such as BPSK or QPSK are utilized. Multiple coding rates may be available for each modulation scheme to provide finer AMC granularity, to enable a closer match between the quality and the transmitted signal characteristics (e.g., R=¼, ½, and ¾ for QPSK; R=½ and R=⅔ for 16-QAM, etc.). AMC typically yields higher system throughputs and higher data rates than other conventional link adaptation techniques such as power control.
The performance of any system employing AMC is highly dependent upon the accuracy in determining a receiver's channel quality, and in particular the accuracy of the underlying link error probability prediction. Link error probability prediction maps the current radio conditions (channel quality) to an expected Frame Error Rate (FER). Bad link prediction severely degrades the performance of AMC. In order to achieve high system throughput, a simple link error probability predictor that accurately models coded OFDM performance or any other multi-carrier modulation is therefore critical for any multicarrier system employing AMC. Therefore, a need exists for a method and apparatus for accurately determining channel quality and performing adaptive modulation/coding within a multicarrier communication system.